1. Field of the Invention
This invention relates to optical fibers having a silicone core. More precisely, this invention relates to optical fibers having a large diameter silicone core capable of bending and twisting as is frequently required in medical and commercial illumination applications.
2. Description of the Prior Art
The use of optical fibers, hereinafter alternatively referred to as "optical waveguides" or simply "waveguides," has become very common since the mid-1970's when Corning first produced a highly pure form of glass which had the ability to transmit light energy. Since then, glass of varying chemical compositions has been developed to transmit light in wavelengths from the ultraviolet to the far infrared regions of the electromagnetic spectrum. Optical waveguides are currently fabricated from various forms of glass and plastic materials such as polymethylmethacrylate (PMMA) and polycarbonate. Such plastic cores are relatively inexpensive to produce when compared with glass and are finding application in the communication industry where flexibility is not a desirable property.
The key to developing an optical waveguide is selecting materials which are highly transparent to the wavelength being transmitted. Optical waveguides have a "core"; that is, an elongate optically transparent path through which light may pass. Conditions must be created to induce the light waves which travel inside of the optical waveguide to remain in the core and not leak out of the sides of the fiber. To that end, a reflective material is coated onto the outside of the core material which acts as a mirror to bend the light back into the fiber core in a process known as total internal reflection (TIR). This outer coating is called the cladding.
The physical properties of light waves are such that as light is transmitted from one medium to another it bends or is refracted slightly at a predictable and repeatable angle. This property is known as the index of refraction. For TIR to occur, the index of refraction of the core material must be greater than the index of refraction of the cladding material. The greater the disparity between the refractive indices of the two materials, the greater the acceptance angle of the fiber input and the greater the divergence of the output. The numerical aperture (N.A.) is the measure of this disparity. EQU N.A.=(n.sup.2 core-n.sup.2 clad).sup.1/2
where n refers to the refractive index of either the core or cladding as indicated.
Since most applications require the output of the fiber to produce a minimally diverging transmission, the most useful fibers have very slight differences between the core and cladding refractive indices, thus a low N.A.
Basically there are three types of fiber optics commercially available for medical application. In each case the core material is glass. Cladding material varies from fiber type to fiber type. The first type of fiber is known as a glass-clad fiber. Usually the material used for the cladding in a glass-clad fiber is the same as the core material but doped with fluorine. The fluorine doping process changes the refractive index of the cladding material only siightly allowing the fiber to have a low N.A. (as low as 0.1).
The second type of fiber is known as plastic clad silica or PCS. The core material is again silica or glass. The cladding of PCS fiber is usually silicone. The silicone material is chosen for its optical properties to produce fibers with an N.A. from 0.3-1.0.
The third and newest type of fiber is known as hard clad silica or HCS. The cladding material of HCS is one of a family of polymers with optical properties which induce TIR. HCS fiber often has a relatively high N.A. (greater than 0.35)
Telecommunication applications dominate the optical fiber market. Kilometers of fiber optic cable are used where copper cable was previously employed. Because conditions are often harsh and the environment often corrosive, protecting the fragile glass core is a primary concern.
Thick buffers and jackets are incorporated into the optical fiber to protect the core. This causes the total diameter of the fiber to become quite large. A large outer diameter (OD) reduces the flexibility of the fiber dramatically (as the diameter of the fiber is doubled, the flexibility of the fiber is reduced by a factor of four), but, as mentioned earlier, flexibility is not a great concern for telecommunication applications.
The requirements for optical waveguides for medical applications are quite different from optical waveguides used for communications. Medical applications often require very short lengths of fiber (less than 5 meters). A medical fiber optic is often disposable (single use), short length, flexible, and has a large core diameter to outer diameter ratio (core/OD). Disposable fibers do not need the bulky buffers and jackets developed for communication which considerably reduce the overall fiber flexibility. Since waveguides are often used with an endoscope, small flexible fibers must be employed. To achieve this flexibility while maintaining a large cross section, multifiber bundles must be used to simulate a large core fiber. This arrangement produces considerable dead space (areas of the cross section which are not occupied by the actively transmitting core). Dead space can account for up to 50% of the cross-sectional area.
To meet the need of the medical community and other fiber markets which require short length, flexible, large core, high core/OD ratio transmission of laser energy, a silicone waveguide has been conceptualized. By proper selection of elastomers, a core and cladding configuration similar to glass fibers can be fabricated to produce a fiber with an N.A. as low as 0.17.
Because silicone is so flexible, a large core waveguide can be produced with a 600 micron core, but able to negotiate tortuous bends similar to a 100-200 micron core glass fiber. The cost of producing such a waveguide would be favorable compared to glass. Such a silicone-cored waveguide would be particularly useful for distributing illuminating light from a central source to remote points as, for example, is desirable in automotive and commercial and residential construction applications.
In summary, the advantages of a silicone waveguide over the prior art glass core fiber optics or waveguides for medical application involving introdution of the waveguide with the body are:
1. Greater flexibility than glass for the same core/OD ratio (especially in a large core waveguide). PA1 2. Shatterproof. PA1 3. Large core/OD ratio possible with much less dead space than bundled glass waveguides of the same total outer diameter with increased flexibility. PA1 4. Costs no more than a glass waveguide.